The present invention relates to a charged particle buncher.
Charged particle bunchers operate to collect charged particles which are spatially dispersed along one or more axes and bring them closer together later in time. A primary application of charged particle bunchers is in time of flight mass spectrometry.
A simple form of charged particle buncher comprises two spaced apart plate electrodes. The electrodes are spaced apart and generally parallel to each other. Each electrode includes an aperture near its center through which charged particles may pass. In use, a group of charged particles drift along an axis extending between the electrodes through the aperture in each electrode. Each electrode is initially held at a first potential. The value of the potential of one of the electrodes is then rapidly adjusted by means of a high-speed switch. For example, initially both electrodes might be held at ground and then the potential of one of the electrodes is rapidly increased to a value V. This generates an electric field between the plates that can accelerate or decelerate charged particles moving between the plates causing them to bunch. In practice, the potential must be changed in a time that is much shorter than the time taken for the charged particles to travel between the electrodes.
Where two flat plate electrodes are used, the electric field is uniform between the plates. Such a field will provide first order bunching of a group of charged particles drifting between the two electrodes of the device.
Higher order bunching can be achieved by generating a non-uniform, or shaped, electric field. EP 0456516 discloses a charged particle buncher for storing ions moving along a path. The buncher is arranged to subject ions to a retarding field during an initial part only of a preset time interval. The field has a spatial variation such that ions that have the same mass-to-charge ratio and enter the buncher during the initial part of the pre-set time interval are all brought to a time focus during the remaining part of the time interval. The retarding field is generated by a plurality of spaced apart hyperboloid electrodes that lie along equipotentials of the retarding field. The electrodes are maintained at the required voltages through being connected together in series with resistors.
It is the applicant's contention that the buncher of EP 0456516 will not operate as described. For satisfactory operation of the buncher, it is necessary to be able to collapse the retarding field at the end of the initial part of the time interval in a time that is much shorter than the transit time for ions to traverse the plurality of electrodes. The applicant believes that the described buncher will not be capable of achieving this because the retarding field is maintained by a conduction current flowing between the plurality of electrodes so that the electric field shape is generated by supporting free charges on intermediate electrodes of the plurality of electrodes. When the field is reduced to zero the free charges have to flow away to ground through the resistors. For a given electrode, this takes a time equal to several times RC where R is the resistance to ground and C is the capacitance of the electrode. In practice, R will be determined by the properties of the power supply and C by the properties of the electrode structure. For a 10 kV, 1 mA supply the minimum value of the resistance will be 10 Megohms, whereas the capacitance of the electrode structure will hardly be less than 10 picofarads. This gives a value for RC of ˜100 μs which is of the order of the transit time for ions traveling through the buncher. This is too long for the device to operate as described.
It is an object of the present invention to overcome, or at least reduce, the above mentioned problem.